Polyphosphate-modified calcium aluminate cement under normal and elevated temperatures: Phase evolution, microstructure, and mechanical properties

This investigation sought to develop a chemically bonded refractory castable based on the acid-base reaction between calcium aluminate cement (CAC) and polyphosphate-based reactant. The mechanical properties, phase evolution, and microstructure of the polyphosphate-modified CAC under normal and elevated temperatures were investigated. The results show that CAC paste had poor fluidity and presented compressive strength decay when submitted to air curing of 20 °C for 28 days compared with that at 7 days. In contrast, the addition of sodium hexametaphosphate (SHMP) significantly improved the fluidity of the resulting binder due to its favorable deflocculating effect for thorough hydration of CAC particles. SHMP-modified CAC paste also exhibited greater compressive strength than CAC paste, and no strength regression occurred after 28 days. Moreover, this superior mechanical performance was maintained even after exposure to elevated temperatures up to 1000 °C. The observed excellent performance in strength was attributed to the enhancement from the phases of calcium aluminate hydrate-based gel and calcium-phosphate, known as chemically bonded ceramics formed by the acid-base reaction between CAC and SHMP. The formed chemically bonded ceramics impeded to some extent the conversion of the hydrates CAH10 or C2AH8 to C3AH6 at ambient temperature and retained a notable bonding effect at high temperatures.